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Population Ecology
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What is a Population?• Population - A group of individuals of the
same species that live together and interbreed
• Populations:o Share resourceso Are influenced by the same environmental factorso Breed with one another
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Density and Distribution• Two important characteristics of a
population are the density, and spacing of individuals
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Population Density• Population Density – Number of
individuals per unit or area of volume• Examples:
o Number of oak tree’s per acreo Number of deer per square mileo Number of plankton per liter of water
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Measuring population density• One of the most popular method of
calculating population density is the mark-recapture method
• Here’s how it is done…
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Steps to the Mark-Recapture Method1.Boundaries are set for the study2.Traps are set out to capture animals3.Animals are marked with tags, dies, collars,
or bands, then released4.Traps are set again at a later date5.Recaptured animals are used to estimate
population density of an area
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FormulaNumber of Recaptures in Second Catch
Total number in Second CatchNumber Marked in First Catch
Total Population (N)
Number Marked in First Catch X Total number in second catch
Number of Recaptures in Second Catch
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Example• Scientists capture 50 rabbits, and put ear
tags on them
• Two weeks later, 100 rabbits are capturedo 10 of these rabbits were recaptures from the first
capture
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FormulaNumber of Recaptures in Second Catch
Total number in Second CatchNumber Marked in First Catch
Total Population (N)
The estimated population would
be 500 rabbits
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Number Marked in First Catch X Total number in second catch
Number of Recaptures in Second Catch
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Other Methods• Scientists will sometimes use indexes for
population size, by counting things such as:o Number of Nestso Number of Burrowso Trackso Droppings
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Dispersion• Population Dispersion – The pattern of
spacing among individuals within the geographic boundaries of the population
• Three main types of population Dispersion:o Clumpedo Uniformo Random
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Clumped• Clumped Dispersion –
When individuals are grouped into patches
• Why would they be clumped?o Food Sources for animalso Soil Conditions for plants
• This is the most common type of dispersion
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Uniform• Uniform Dispersion – When individuals
are evenly spaced
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Random• Random Dispersion – Unpredictable
distribution of individuals• The most rare of the three types of dispersion
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Demography• Demography – The study of the vital
statistics that affect population size• Some Demographic Variables:
o Birth Rateo Mortality Rateo Immigration Rateso Emigration Rates
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Life Tables• Life Tables – An age-specific summary of the
survival pattern of a population• Life Tables Show:
o Probability of surviving a particular ageo Life expectancyo Fertility at each age
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Uses of Life Tables• Originally created for life insurance agencies• Biologists use them to study population
dynamics• See if mortality rates increase due to
environmental factors• See at what age a species is most fertile
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Survivorship Curves• Survivorship Curves – A plot of the
proportion or numbers in a cohort still alive at each age
• There are 3 types of survivorship curveso Io IIo III
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Type I• Little mortality at early ages• Mortality rate increases as the organisms
get older• An example of this is with humans• We have few offspring, but give them great
care
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Type III• This curve shows high mortality rates during
the early stages of life• The line flattens out once the organisms
reach a ‘critical age’ of survival• This is for animals that produces many
offspring, but little to no care for themo Example – plants, clams, fish
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Type II• This is a constant death rate• This falls in-between I and III type curves• Examples here are squirrels, lizards
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Population Growth• Biotic Potential – the maximum growth rate
of a population under ideal conditions, and unlimited resources (no restrictions)
• Factors affecting biotic potential:
• Age of Reproductive Maturity
• Clutch Size• Frequency of
Reproduction
• Reproductive Lifetime• Survivorship of offspring
to reproductive maturity
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Carrying Capacity• The maximum number of individuals of a
population that can be sustained in a particular habitato How many rainbow trout could this creek
support?o How many wolves can survive in Isle Royale
National Park?
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Limiting Factors• Limiting factors are things that prevent
populations from reaching their biotic potential (2 types)
• Density-Dependent Factors• Density-Independent Factors
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Density-Dependent Factors• Factors whose effects become more intense as
the pop. density becomes greater• Examples:
o Resource Competition (Food, Space, Sunlight, O2)o Toxic Levels of Waste (CO2 levels)
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Density-Independent Factors• Factors that are not effected by pop. density • Examples:
o Natural Disasterso Climate extremeso Pollution
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Population Formulas• Population Growth Formula:
r = reproductive rateN = population size
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Growth Patterns• There are 2 types of pop. Growth patterns:• Exponential – Occurs when growth rate is
above zero• Logistical – When limiting factors restrict
size of population
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